This invention relates to a quantization apparatus and, more particularly, to a quantization apparatus in which digitized stereo input signals are processed with quantization and word length limitation.
In certain prior-art apparatus for quantization, a dither addition circuit is provided for improving reproducibility by alleviating dropout of the information of weak intensity signals produced on quantization and word length limitation.
With such quantization apparatus, as disclosed in JP Patent Kokai (laid-Open) Patent Publication No. 05-145376 (1993), a dither addition circuit is provided upstream of a quantizer for adding dither signals to digital data in order to prevent failure in the waveform or level shifting and consequent deterioration in reproducibility due to word length limitation by rounding or half-adjustment during quantization of digital data by the quantizer and consequent dropout in the information contained in substantially sinusoidal pre-quantization weak-intensity signals. In this case, if, after quantization of the dither signals added to the digital data, a pre-set number of the lower bits are rounded or half-adjusted, the information proper to the minute or weak-intensity signals contained in the input signal is left in the quantized data for further alleviating the failure in the information of the minute weak-intensity signals induced by the word length limitation.
If, when the right-channel digital stereo signals and the left-channel digital stereo signals are supplied to a quantizer for the right channel and to a quantizer for the left channel, respectively, the same dither signals or dither signals not correlated with each other are supplied to left-channel and right-channel dither addition circuits provided upstream of the quantizers, the correlation between the left and right channels, proper to the stereo input signals, is deteriorated.
For example, if the same dither signals, having the cross-correlation coefficient equal to unity, are supplied to the left and right dither addition signals, the cross-correlation of signal components having inherently low left channel--right channel correlation is increased. Specifically, the ambience feeling created by the reverberating stereophonic components in music signals is not spread sufficiently towards left and right, but is collected towards a center position.
On the other hand, if the dither signals not correlated with each other, such as the dither signals having the cross-correlation coefficient equal to zero, are supplied to the left and right dither addition circuits, the cross-correlation of signal components having the left channel-right channel correlation coefficient equal to unity is decreased. Specifically, the sound image of the sound having a fixed center sound source position feeling becomes; blurred and spread toward left and right.